Aldosterone is a steroid hormone secreted from the adrenal gland which binds and activates the mineralocorticoid receptor (MR). In the primary cells of the distal tubules and collecting ducts of the kidney, MR activation leads to sodium and water retention with excretion of potassium resulting in plasma volume expansion leading to increased blood pressure (BP). Excess aldosterone measured in circulation is termed primary aldosteronism (PA) and occurs when aldosterone production is dysregulated by the renin-angiotensin-aldosterone system (RAAS). PA was initially identified in patients with adrenal adenomas with recent evidence suggesting an increase in prevalence associated with obesity. PA is a common cause of secondary hypertension with the prevalence of PA ranging from 14-21% in patients with resistant hypertension (RHTN), a condition defined as BP remaining above goal despite the concurrent use of 3 antihypertensive agents of different classes including a diuretic agent. Recent studies have shown an association between excess aldosterone, RHTN, and obstructive sleep apnea (OSA) which is worsened by aldosterone-mediated fluid retention.
Local overproduction of aldosterone has been noted in several severe disease states even when no significant plasma elevation is observed. In patients with chronic congestive heart failure (CHF), aldosterone levels in failing heart tissue is higher than in peripheral plasma. In animal models of kidney disease, local production of aldosterone in the renal cortex is postulated to contribute to disease progression. In both these states, local elevated aldosterone levels contribute to harmful effects via both MR-dependent and MR-independent mechanisms including the generation of reactive oxygen species and endothelial dysfunction leading to inflammation and stimulation of cell growth and proliferation with upregulated collagen deposition leading to fibrosis.
Antagonists of MR, including spironolactone and eplerenone, have been extensively used to block the effects of aldosterone binding to MR. Significant reductions in morbidity and mortality in patients with heart failure or myocardial infarction have been demonstrated with these agents in combination with angiotensin-converting enzyme (ACE) inhibitors and diuretics (RALES & EPHESUS trials). Side effects including hyperkalemia are seen with both agents with the nonselective spironolactone also eliciting gynaecomastia via nonselective modulation of the progesterone and androgen receptors. Additionally, elevations of renin and aldosterone result from MR antagonism and thus the MR-independent (non-genomic) effects of aldosterone are exacerbated.
In contrast to MR antagonists, inhibition of CYP11B2 (aldosterone synthase), the key enzyme in aldosterone biosynthesis, should afford the beneficial effects of MR antagonism without the deleterious buildup of aldosterone leading to activation of MR-independent inflammatory and fibrotic states. CYP11B2 is a mitochondrial cytochrome P450 enzyme which which converts 11-deoxycorticosterone to aldosterone. Selective inhibition of CYP11B2 represents a promising treatment for aldosterone related diseases.
The highly homologous metalloenzyme CYP11B1 (11-β-steroid-hydroxylase) catalyzes the formation of the primary glucocorticoid cortisol from 11-deoxycortisol. Given the high degree of homology between CYP11B2 and CYP11B1 (93%), the development of selective CYP11B2 inhibitors has been a significant challenge. The inhibitor Osilodrostat (LCI-699) was developed as a CYP11B2 inhibitor for the treatment of hypertension but was abandoned due to its potent inhibition of CYP11B1. Selective compounds which block the production of aldosterone via CYP11B2 without inhibition of cortisol production via CYP11B1 are described herein.
Living organisms have developed tightly regulated processes that specifically import metals, transport them to intracellular storage sites and ultimately transport them to sites of use. One of the most important functions of metals such as zinc and iron in biological systems is to enable the activity of metalloenzymes. Metalloenzymes are enzymes that incorporate metal ions into the enzyme active site and utilize the metal as a part of the catalytic process. More than one-third of all characterized enzymes are metalloenzymes.
The function of metalloenzymes is highly dependent on the presence of the metal ion in the active site of the enzyme. It is well recognized that agents which bind to and inactivate the active site metal ion dramatically decrease the activity of the enzyme. Nature employs this same strategy to decrease the activity of certain metalloenzymes during periods in which the enzymatic activity is undesirable. For example, the protein TIMP (tissue inhibitor of metalloproteases) binds to the zinc ion in the active site of various matrix metalloprotease enzymes and thereby arrests the enzymatic activity. The pharmaceutical industry has used the same strategy in the design of therapeutic agents. For example, the azole antifungal agents fluconazole and voriconazole contain a 1-(1,2,4-triazole) group that binds to the heme iron present in the active site of the target enzyme lanosterol demethylase and thereby inactivates the enzyme. Another example includes the zinc-binding hydroxamic acid group that has been incorporated into most published inhibitors of matrix metalloproteinases and histone deacetylases. Another example is the zinc-binding carboxylic acid group that has been incorporated into most published angiotensin-converting enzyme inhibitors.
In the design of clinically safe and effective metalloenzyme inhibitors, use of the most appropriate metal-binding group for the particular target and clinical indication is critical. If a weakly binding metal-binding group is utilized, potency may be suboptimal. On the other hand, if a very tightly binding metal-binding group is utilized, selectivity for the target enzyme versus related metalloenzymes may be suboptimal. The lack of optimal selectivity can be a cause for clinical toxicity due to unintended inhibition of these off-target metalloenzymes. One example of such clinical toxicity is the unintended inhibition of human drug metabolizing enzymes such as CYP2C9, CYP2C19 and CYP3A4 by the currently-available azole antifungal agents such as fluconazole and voriconazole. It is believed that this off-target inhibition is caused primarily by the indiscriminate binding of the currently utilized 1-(1,2,4-triazole) to iron in the active site of CYP2C9, CYP2C19 and CYP3A4. Another example of this is the joint pain that has been observed in many clinical trials of matrix metalloproteinase inhibitors. This toxicity is considered to be related to inhibition of off-target metalloenzymes due to indiscriminate binding of the hydroxamic acid group to zinc in the off-target active sites.
Therefore, the search for metal-binding groups that can achieve a better balance of potency and selectivity remains an important goal and would be significant in the realization of therapeutic agents and methods to address currently unmet needs in treating and preventing diseases, disorders and symptoms thereof.